Decahydroisoquinoline carboxylic acid derivatives, which may be represented by the formula ##STR1## where R is a group which together with the carbonyl forms an acid, acid derivative or ketone, such as OH, NH-alkyl, O-alkyl, S-alkyl, alkyl, aryl, and R.sup.1 and R.sup.2 independently represent a group or arrangement such as hydroxyl, ketonic oxygen, alkyl or aryl, or when adjacently positioned are arylene or alkylene such as phenylene or 1,4-tetramethylene, are useful as intermediates in the preparation of biologically active compounds. There are many literature references to these compounds. Among the applications is their use in the preparation of N-methyl-D-aspartic acid antagonists as reported by Ornstein, P. L. et al in J. Org. Chem, 1991, 56, 4388. The structure shown in a recent publication of a compound potentially useful in the treatment of Acquired Immune Deficiency Syndrome (AIDS), has as part of its structure, N-tert-butyldecahydroisoquinoline-3-carboxamide. Roberts, N. A. et al, Science 1990, 248,358.
A previously reported synthesis for the preparation of N-tert-butyldecahydroisoquinoline-3-carboxamide is that of Hayashi, K. et al., Chem. Pharm. Bull, 1983, 31(1), 312, which employed the Pictet-Spengler reaction. This reaction uses L-phenylalanine and formalin in concentrated hydrochloric acid at 95.degree. C. Under these conditions the tetrahydroisoquinoline-3-carboxylic acid derivative obtained had suffered epimerization at the C-3 position. Four steps were required for the resolution of the product so obtained to the required 3-S enantiomer; subsequent hydrogenation of the phenyl ring to the desired decahydroisoquinoline-3-cazboxylic acid produced four possible diastereomers from which the desired diastereomer could only be obtained in a low overall yield. It is desired therefore to find a means for stereospecifically producing decahydroiso-quinoline-3-carboxylic acid and for producing it in a greater yield.
It is a particular object of the present invention to provide a method for producing optically. pure N-tert-butyl-decahydro-4aS,8aS-isoquinoline-3S-carboxamide in good yields.